In satellite imaging of the Earth's surface, it is often desirable to know the wavenumber (or wavelength) content of the reflected and emitted light, since this can indicate composition of the reflecting or emitting material. It is typical in such an application to restrict attention to optical and near IR regions of the spectrum, that is, to wavelengths in the range of about 400 nm to 2400 nm. When a moderate number of wavelength bins (ie. 5 to 20) are observed, this is called “multispectral imaging.” When a large number of such bins (ie. 200) are observed, it is called “hyperspectral imaging.” Hyperspectral imaging is characterized by an average wavenumber bin size of about 10 nm.
In one imaging spectroscopy approach, light from a single line in the image is split into component wavelengths by a prism or grating. Such an approach requires that each line of the image be acquired separately, and the signal-to-noise ratio (SNR) of each measured component will be worse than if the panchromatic image were recorded. Another imaging spectroscopy approach uses a piece of equipment called a Michelson interferometer and a computational technique known as Fourier Transform Imaging Spectroscopy (FTIS). In this approach, the light that would be measured to form an image is split, and the two versions are delayed with respect to each other over a range of delays, producing an amplitude-modulated panchromatic image for each delay. This has the effect of producing the sampled autocorrelation function of the light at each image location, and a point-by-point Fourier transform computation recovers the spectrum at each such location. In this approach, the mass of the interferometer reduces the total mass of other equipment that can reside on the orbital platform, some of the light is lost in the splitting operation of the interferometer, and some of the component panchromatic images can have very low SNR, which can affect the quality of the resulting hyperspectral image. In yet another approach, the effect of the interferometer is obtained by moving one portion of an optical array relative to another; this approach uses the same FTIS computation as is used with the Michelson interferometer. This approach has the advantage of eliminating the interferometer, but it limits the high spatial frequency content of the hyperspectral image to be less than that which could be obtained using the Michelson interferometer. This has the undesirable effect of removing some scene detail from the hyperspectral image.
It is desirable to provide an approach to imaging spectroscopy that does not use a Michelson interferometer or prism, thus saving payload mass, and yet reproduces any spatial frequencies passed by the optical imaging system in the reconstructed hyperspectral image.